Spouted bed blending is a blending technique whereby particulate material is circulated pneumatically within a blending vessel. Typically, the material is intermixed and transported from the bottom of the vessel to an area over the top of the powder material bed within the vessel by transferring the material through a draft tube. A deflector positioned at the top of the draft tube splits the powder stream and the material settles by gravity, recombining with the powder material bed within the vessel. Pneumatic circulation produces a gentle action with low shear forces in the particulate powder material.
Nuclear fuel powder material blending requires that both a macro and micro homogeneity specification be met by the material blend. Certain nuclear oxide powders tend to form soft agglomerates which may exceed the micro homogeneity specifications for the manufacture of nuclear fuel pellets. The typical prior art processing required a ing to destroy any such soft agglomerates.
Macro homogeneity is based on a one gram sample. For a typical blending specification of 6% plutonium oxide, PuO.sub.2, concentration in a uranium dioxide, UO.sub.2, matrix the chi squared distribution limits for 95% of values fall within a desired limit range of 0.19% and -0.05% in relation to different enrichments, and taking the 0.19+0.05 to give 0.24% which is then divided by 2 to provide a workable form for various enrichments of a 2% relative error. Thusly, it was established that 2% of the nominal enrichment as a relative error is allowed to meet the desired macro homogeneity. Then by taking x number of samples and analyzing these samples for plutonium oxide, would give the standard deviation. The chi squared tables permit the calculation of the standard deviation at the 95% confidence levels and these are corrected for analytical error. This valve is then divided by the sample mean to give the relative error which can be compared to the 2% target value.
Micro homogeneity involves the size of the enriched domains within a fabricated fuel pellet and is determined by polishing a fuel pellet cross section and microscopically analyzing the polished surface. A typical micro homogeneity specification is that no domain of plutonium oxide having an effective diameter larger than 400 microns exists anywhere in the fuel pellet. Failure to meet this specification could result in hot spots within the pellet during operation which could cause fuel pellet failure.
There are two known types of spouted bed blenders. One utilizes an unconfined draft induced by gas jets at the bottom of the powder material bed such as disclosed by U.S. Pat. No. 3,746,312 of Pirk et al, and the other includes a draft tube or tubes through which one or more gas jets issue providing for the increased mixture of the powder material being blended.
The typical nuclear fuel pellet powder material has a particle size of 10 microns or less and tends to agglomerate due to electrostatic charge accumulation. Such agglomeration interferes with achieving the fabrication requirements for mixed oxide fuel pellets relative to macro and micro homogeneity. Because of the fineness of this powder material, a conventional fluidized bed apparatus is not suitable for blending purposes since the fluidizing gas produces channels in the fine powder material through which the gas passes resulting the stagnant zones on either sides of said channels and formation of agglomerates in the powder material.
Mechanical mixers with a rotating member inside the mixer are generally not acceptable for blending large batches of nuclear fuel powders, i.e., greater than 150 kilograms, because of the geometry constraints for criticality control throughout the nuclear fuel blending and fabricating processes.